This invention relates to a radio communication apparatus which is capable of transmitting a transmission data signal and receiving a reception data signal and, in particular, to the radio communication apparatus which carries out communication in a manner of a time division multiple access (TDMA)-TDD (Time Domain Duplex). Such communication will be simply referred to as TDMA-TDD communication hereinunder.
In such TDMA-TDD, it is to be noted that a common carrier frequency ,wave is used as both a transmission carrier frequency wave and a reception carrier frequency wave and that communication is carried out in a time division fashion.
A conventional radio communication of the type described comprises a receiver and a transmitter which are used for the TDMA-TDD communication and which receive a reception data signal and transmit an input data signal, respectively. The radio communication apparatus is selectively put into a reception state or a transmission state so as to carry out the TDMA-TDD communication. In other words, the transmitter and the receiver are alternately put into an active state and an inactive state. This shows that a selected one of the transmitter and the receiver is alone put into the active state during the communication.
In the meanwhile, quadrature phase shift keying (QPSK), especially, .pi./4-shift QPSK, has been often used in the transmitter of the TDMA-TDD communication to modulate a data signal and to transmit a transmission radio wave subjected to the QPSK. Such a modulated signal which is subjected to the QPSK in the above-mentioned manner is received as a reception radio wave by the receiver.
In the radio communication apparatus for the TDMA-TDD communication, a common local frequency is also used as a local transmission frequency and a local reception frequency so as to carry out the transmission and the reception operations by the use of the common carrier frequency wave in the transmitter and the receiver, respectively.
With the radio communication apparatus, it is preferable that the transmitter is completely electromagnetically isolated from the receiver while the reception operation is carried out in the receiver with the transmitter kept inactive. Stated otherwise, the transmission radio wave should be sufficiently attenuated during the reception operation. Otherwise, the transmission radio wave signal adversely influences the reception radio wave because both the transmission and the reception radio waves include a common frequency component. Such an adverse influence also occurs from using the common local frequency in the transmitter and the receiver. This results in degradation of a reception characteristic in the receiver.
Herein, recent requirements have been to make the radio communication apparatus small in size and light in weight. Under the circumstances, complete isolation can not be kept between the transmitter and the receiver of the above-mentioned radio communication apparatus. Therefore, the reception characteristic is inevitably degraded in the conventional radio communication apparatus.
In order to achieve sufficient isolation between the transmitter and the receiver, consideration might be made about putting a modulation circuit of the transmitter into an inactive state by switching off a modulation local oscillator of the modulation circuit. However, such a modulation local oscillator is slowly switched on after it is once switched off. This means that such a radio communication apparatus is unsuitable for the TDMA-TDD communication because the reception and the transmission operations should be changed from one to another within a very short time interval in the TDMA-TDD communication.